An example of conventional dual wavelength semiconductor laser device is disclosed in Patent Document 1. The laser device includes an infrared laser emitting element and a red laser emitting element, both arranged side by side on a single substrate. Such a semiconductor laser device is said to have a monolithic structure. The infrared laser emitting element and the red laser emitting element emit laser beams of different wavelengths.
The infrared laser emitting element has a laminated structure made up of an n-type first lower clad layer, a first active layer and a p-type first upper clad layer. The red laser emitting element has a laminated structure made up of an n-type second lower clad layer, a second active layer and a p-type second upper clad layer.
The semiconductor laser device is manufactured as follows. First, a first laminate is formed over the upper surface of a substrate. The first laminate is made up of an n-type first lower clad layer, a first active layer and a p-type first upper clad layer, where these layers are laminated one upon another in the mentioned order. Then, a first etching step is performed to remove part of the first laminate other than the portion which is to become an infrared laser emitting element. As a result, the upper surface of the substrate is exposed except the portion on which the infrared laser emitting element is to be formed.
After the first etching step, a second laminate is formed on both the exposed portion of the upper surface of the substrate and the portion to become the infrared laser emitting element. The second laminate is made up of an n-type second lower clad layer, a second active layer and a p-type second upper clad layer, where these layers are laminated one upon another in the mentioned order. Then, a second etching step is performed to remove part of the second laminate other than the portion which is to become a red laser emitting element.
Patent Document 2 discloses another conventional semiconductor laser device having a different structure. This semiconductor laser device includes an embankment layer (height adjustment buffer layer) formed between the substrate of a red laser emitting element and the second lower clad layer. With this feature, the semiconductor laser device disclosed in Patent Document 2 differs from the device disclosed in Patent Document 1.
The semiconductor laser device of Patent Document 2 is manufactured as follows. First, a first laminate is formed over the upper surface of a substrate. The first laminate is made up of an n-type first lower clad layer, a first active layer and a p-type first upper clad layer, where the these layers are laminated one upon another in the mentioned order. Then, a first etching step is performed to remove part of the first laminate other than the portion which is to become an infrared laser emitting element. As a result, the upper surface of the substrate is exposed except the portion on which the infrared laser emitting element is to be formed.
After the first etching step, an embankment layer (height adjustment buffer layer) is formed on the exposed portion of the upper surface of the substrate. Then, a second laminate is formed on the embankment layer and on the portion to become the infrared laser emitting element. The second laminate is made up of an n-type second lower clad layer, a second active layer and a p-type second upper clad layer, where these layers are laminated one upon another in the mentioned order. Then, a second etching step is performed to remove part of the second laminate other than the portion which is to become a red laser emitting element.                Patent Document 1: JP-A-2001-244569        Patent Document 1: JP-A-2001-320132        
In the above semiconductor laser devices, circumstances relating to e.g. the assembling process or desired optical properties necessitate substantially equal heights for the first active layer of the infrared laser emitting element and the second active layer of the red laser emitting element, as measured from the upper surface of the substrate.
Meanwhile, the second lower clad layer of the red laser emitting element differs in composition from the first lower clad layer of the infrared laser emitting element. This difference permits the second lower clad layer to be made considerably thinner than the first lower clad layer.
In the manufacturing method disclosed in Patent Document 1, the thickness of the second lower clad layer of the second laminate is made equal to that of the first lower clad layer of the first laminate, so that the first active layer and the second active layer have substantially equal heights. While the second laminate can be made thinner as noted above, the thickness of the second lower clad layer is made considerably large for equalizing the heights of the first active layer and the second active layer. Accordingly, the film formation for forming the second lower clad layer requires a lot of time.
Moreover, in the manufacturing method described in Patent Document 1, a large amount of material is required for making the thick second lower clad layer, which leads to a considerable increase in the manufacturing cost.
In the manufacturing method of Patent Document 2, on the other hand, the embankment layer is formed on the portion exposed by the first etching, and on the upper surface of this embankment layer is formed the second lower clad layer. In this manner, the thickness of the second lower clad layer can be reduced by as much as the thickness of the embankment layer. In addition, by adjusting the thickness of the second lower clad layer, the first active layer and the second active layer can be made substantially equal in height.
However, the manufacturing method of Patent Document 2 requires the additional film formation step for forming the embankment layer, performed after the first etching step for forming the first laminate. Unfavorably, the additional step leads to a considerable increase in the manufacturing cost.